This is a proposal to acquire a state-of-the-art Beckman Optima(TM) XL-I analytical ultracentrifuge equipped with both absorbance and Rayleigh interference optics. There are currently no analytical ultracentrifuges available at Tufts University on any of the three campuses. The investigator group is composed of seven users, six of whom are funded by the NIH and will serve as the core user group. Five of the seven investigators are from four different departments at Tufts A & S, and Engineering (Medford) and Tufts University School of Medicine (Boston). Two of the investigators are in the Department of Neurology at Harvard Medical School. The Director of the Analytical Ultracentrifugation Research Laboratory at the Boston Biomedical Research Institute will serve as an external advisor to the facility. In order to accomplish the aims of the NIH-funded research programs at Tufts and Harvard Medical School, the acquisition of an analytical ultracentrifuge has become essential. The need of these researchers for an analytical ultracentrifuge cannot be met under prevailing conditions. The user group identified here brings a broad spectrum of research backgrounds and expertise and is actively engaged in quantitatively probing macromolecular structure and function. Both equilibrium and velocity sedimentation will be used to determine thermodynamic and hydrodynamic parameters of molecular systems. The XL-I will greatly enhance the ability of the major users to determine descriptors associated with macromolecular interactions such as equilibrium constants, stoichiometries and molecular shapes. The analytical ultracentrifuge is uniquely suited to measure properties of macromolecules in solution as a function of pH, ionic strength and solution components. This instrument will fulfill an acute need of NIH supported investigators at Tufts and Harvard Medical School and greatly enrich the existing research infrastructure at Tufts.